The present invention relates to emergency brake control systems and more particularly to such systems used in hoists such as mine hoists employed to transport a conveyance between the ground and the surface level.
Generally, hoists of the type to which the invention pertains include a rotating drum driven by a motor, and a conveyance attached to the drum by means of a cable that wraps around the drum as it rotates in one direction to raise the conveyance and unwraps as the drum rotates in the other direction to lower the conveyance. Occasionally emergency situations, such as over-speed or over-travel of the conveyance, may arise which require stopping the hoist immediately. Emergency braking system are therefore included in the control systems of such hoists.
Existing emergency braking systems provide for immediate stopping of the rotating drum regardless of the speed or direction of travel of the conveyance. While these relatively simple and uncomplicated systems do stop the conveyance in emergency situations, they are undesirable in systems in which deceleration rates of the conveyance must not exceed a maximum rate. Problems with deceleration rates may arise, for example, in the hoisting or ascending mode where the effect of gravity and emergency braking occurring simultaneously may exceed maximum allowable deceleration rates.
For example, instantaneous braking of the conveyance during ascent would stop the conveyance but the upward inertia of the equipment in the conveyance could cause it to rise off the conveyance floor; then it would crash back to the floor, causing possible damage to the equipment and the conveyance.
Existing braking control systems that have proved to do an adequate job of stopping the conveyance in emergency situations within allowable deceleration limits have included relatively complex and expensive electronic circuitry to slow down and bring the conveyance to a stop according to a predetermined program of deceleration rate. This circuitry is responsive to the speed of the conveyance and causes adjustments in the speed of the drum to be made to maintain the deceleration rate of the conveyance within prescribed limits. While the use of such complex electronic circuitry can be justified in some hoists, they are often too expensive for practical use in many hoisting applications, must occasionally be calibrated and adjusted, and are subject to human error or tampering.